One sure sign of an urban environment is the presence of parking decks. According to the U.S Census, the U.S. had 12,573 commercial parking lots and garages at last count (in 2014), with one-third of those estimated to be surface parking lots. That leaves 8,000 or more parking decks, not including all the private parking decks.
Like any type of infrastructure subject to wear and tear, parking decks require routine maintenance and repairs. But they have much more demanding durability requirements than most other buildings because they must withstand harsh weather, water infiltration, and in many areas deicing salts.
“As a result of these natural elements, a parking structure is more likely to deteriorate faster than a typical building,” writes Kathy Phillips, senior vice president of Alliant Insurance Services, on the National Parking Association website. She also notes “the concrete floor slab is the most vulnerable portion of a parking structure and requires the most attention and maintenance.”
Regular power washing is one maintenance activity recommended by Michael Nichols, president of Next Parking, which manages parking properties in Chicago and several other cities. In a 2009 article in Parking magazine, Nichols wrote, “Over time, the use of salt, and even the mixture of salt and other products brought into the garage from the outside, will break down the concrete, causing damage that can be expensive to repair. This, among other reasons, is why regular power washing and mechanical sweeping of a facility is essential.”
However, Nichols also notes that it frequently is tempting to delay such expenses, and that “a pattern of deferred or neglected maintenance will result in a facility with significant issues.” This has certainly contributed to the growth in demand for concrete repairs, but also has pushed the industry to maximize the durability of repair solutions.
The concrete industry has learned a great deal recently about making effective repairs, starting with a better understanding of how they come to be needed and complemented by high-performance repair materials.
“Repair has evolved into more than taking care of whatever is obvious,” said Nam Shiu, senior vice president of Walker Restoration Consultants. A structural engineer who started his career doing research at the Portland Cement Association, followed by nine years honing his troubleshooting skills at Wiss, Janney, Elsner, Shiu has been with Walker since 1998. “The key to repairing concrete is always understanding the underlying mechanism, then designing a repair that will last,” he said. “That way it all comes together as a holistic way of addressing structural issues.”
Dealing with corrosion
For example, corrosion of the reinforcing steel is one of the most common root causes of concrete distress in parking structures. When that is known to be the case, there are several ways to approach the repair, all based on understanding the failure mechanism.
“We can do the minimum—take out the corrosion, clean the rebar, and put the concrete back the way that it was,” Shiu said, adding that the corrosion is likely to continue in the adjacent concrete due to a ring, or halo, effect. Although this is not necessarily a good long-term solution, it is less expensive than other options and may sometimes be appropriate, such as for short-term life extension.
One way to achieve a longer lasting repair is to protect the steel, once it has been exposed, with an epoxy coating, which stops the chloride from reaching the steel and thus prevents corrosion. Adding a corrosion inhibitor to the concrete mortar will further neutralize the chlorides, and using a sealer such as silane can help by keeping water out of the concrete.
“Corrosion is an electrical process,” Shiu explained. “So if you keep water out, there is no electrolyte to pass the current, the electrons, so the corrosion slows down.” Applying a waterproofing membrane to the completed repair can similarly help combat corrosion. Choosing an appropriate coating, based in part on the type of structure, is important to ensure that it improves concrete performance rather than sealing in moisture and hastening the deterioration.
“I’ve always preferred to use vapor permeable systems to allow the concrete in the underlying structure to breathe,” said George Reedy, national director of sales for the Miracote division of Crossfield Products. Reedy has been involved in the concrete protection and repair industry since 1979, with experience in both manufacturing and contracting. In 1988, he was one of the founders of the International Concrete Repair Institute.
Addressing the halo effect in the concrete surrounding the repair through passive cathodic protection can further extend the life of the repair. This approach uses a discrete galvanic anode made of zinc embedded in the concrete to attract the corrosion, diverting it from the reinforcing steel.
Because these repaired areas represent a relatively small portion of the structure, typically less than 10%, Shiu recommends proactive corrosion protection for the rest of the structure as well. At a minimum, a waterproofing membrane should be applied to the entire deck. It can be made even more effective by spraying a migrating corrosion inhibitor (MCI) on the concrete before applying the membrane. As the name implies, the MCI migrates into the concrete and ultimately forms a protective surface on the reinforcing steel.
Reedy says another approach is to apply a colloidal silicate sealer to the concrete. Although somewhat similar to an MCI in that it migrates deep into the substrate, the colloidal silicate chemically reacts with free calcium hydrate compounds in the concrete, converting those compounds into insoluble calcium-silicate-hydrate gel, which elevates the pH and repassivates the reinforcing steel.
The U.S. Bureau of Reclamation in 2014 published its M-82 Standard Protocol to Evaluate the Performance of Corrosion Mitigation Technologies in Concrete Repairs specifically for determining the effectiveness of such treatments.
“In the M-82 testing, the particular colloidal silicate that we use to treat the concrete before we apply a coating was found to be able to purge the equivalent of 40 pounds of chloride per cubic yard of concrete in 24 hours after application,” Reedy said. “That effectively mitigates the corrosion at a 95% confidence limit. To ensure repairs are long-term and sustainable, we want to improve and enhance the concrete, then follow up with a coating to keep out further chlorides in the future and allow any moisture in the slab to escape.”
Where reinforcing steel corrosion is not well controlled by other means, a system of active cathodic protection can be applied. Unlike the passive system that relies on naturally occurring galvanic action, an active cathodic protection system uses an external power source to apply electrical potential and reverse the electrochemical process of corrosion. By driving the current in the direction opposite to its natural flow, an active system can stop and even reverse the corrosion of the steel. Their use on parking structures is not widespread, primarily because they require a lot of maintenance, which drives up cost.
Dealing with thermal issues
It the temperate zone between the northern and southern states, freeze/thaw damage to parking decks can become a concern. Even where air entrainment was included in the original concrete, freeze/thaw cycles can still cause severe damage when the concrete is supersaturated. Therefore, again, keeping the concrete dry by applying a waterproof membrane can help avoid this damage.
Especially in parking decks located further south, wide thermal fluctuations also can be a cause of cracking in concrete. The cracking allows water to get into the concrete, which accelerates corrosion, so properly repairing cracks is important to the concrete’s overall health.
Water entering the concrete through cracks can be especially problematic in the case of post-tensioned and prestressed concrete. Some earlier strand protection schemes have not performed as well as anticipated. When a crack allows water to reach a strand and corrosion begins, a phenomenon called stress corrosion kicks in. These strands under high tension corrode at a rate that is 10 times the corrosion rate of conventional reinforcing steel. Fortunately, in this area as in others, the industry has learned along the way and improved strand protection systems. However, structures built before the 1990s using prestressing and post-tensioning may require special attention in the case of concrete repairs.
Controlling water makes repairs last longer
The art and science of concrete repairs are helping extend the useful lives of more parking decks every year, an important trend in a day when sustainability is frequently a prime concern. In most cases, water is a the root of the problem, whether it concerns corrosion, freezing and thawing, or both. By determining the root cause of the deterioration, selecting the right repair products for the type of parking structure, enhancing the existing concrete to its maximum potential and repassivating steel, and following up with a waterproof membrane, contractors can ensure their concrete repairs are effective, durable for the long term, and sustainable.